1. Field of the Invention
The present invention relates to a semiconductor device, and specifically to a light emitting device using a gallium nitride compound semiconductor.
2. Description of the Related Art
III-V group gallium nitride compound semiconductors represented by, for example, gallium nitride (GaN), indium nitride (InN), and aluminum nitride (AlN) have a wide band gap of 1.9 to 6.2 eV and are expected to be used as materials for light emitting and light receiving devices covering the visible to ultraviolet light bands. A promising candidate of such a light source used for a super-high density optical disc which has been a subject of active research and development is a blue-violet semiconductor light emitting device for emitting light having an oscillating wavelength around 400 nm which can be realized by such materials.
In the field of gallium nitride compound semiconductor light emitting devices, it is conventionally known to use In.sub.x Ga.sub.1-x N (0.1.ltoreq.x.ltoreq.0.2) for an active layer in order to provide laser light having a wavelength of around 400 nm. It is also conventionally known to use Al.sub.y Ga.sub.1-y N (0.ltoreq.y.ltoreq.1) for a clad layer as a material having a refractive index smaller than that of the active layer, in order to effectively confine the light.
Such a gallium nitride compound semiconductor light emitting device is conventionally fabricated to include a sapphire (Al.sub.2 O.sub.3), silicon carbide (SiC) or gallium nitride substrate by using metal organic chemical vapor deposition (MOCVD)
Hereinafter, a conventional gallium nitride compound semiconductor light emitting device will be described with reference to FIG. 4.
FIG. 4 is a cross-sectional view of the conventional gallium nitride compound semiconductor light emitting device. As shown in FIG. 4, the gallium nitride compound semiconductor light emitting device includes a sapphire substrate 1 and the following layers sequentially provided on the sapphire substrate 1: a buffer layer 2 formed of GaN, an n-type contact layer 3 formed of n-type GaN, an n-type clad layer 4 formed of n-type Al.sub.y Ga.sub.1-y N (0&lt;y1.ltoreq.1), an active layer 5 having a multiple quantum well structure including a plurality of In.sub.0.15 Ga.sub.0.85 N well layers (not shown) and a plurality of In.sub.0.05 Ga.sub.0.95 N barrier layers (not shown) which are alternately layered, a p-type clad layer 6 formed of p-type Al.sub.y2 Ga.sub.1-y2 N (0&lt;y2.ltoreq.1), and a p-type contact layer 7 formed of p-type GaN. The gallium nitride compound semiconductor light emitting device further includes an n-type ohmic electrode 8 provided on the n-type contact layer 3 and a p-type ohmic electrode 9 provided on the p-type contact layer 7, as shown in FIG. 4.
The buffer layer 2 is provided for alleviating the lattice mismatch between the sapphire substrate 1 and the layers 3 through 7 formed of the gallium nitride compound semiconductors, especially, the n-type contact layer 3, and for improving the crystallinity of the layers 3 through 7 formed of the gallium nitride compound semiconductors. The n-type contact layer 3 usually has a thickness of about 2 .mu.m or more in order to further improve the crystallinity of the layers 3 through 7.
In order to sufficiently confine light generated in the active layer 5, the Al ratio (the ratio of Al to the other elements in the compound) or the thickness of both the n-type clad layer 4 and the p-type clad layer 6 need to be increased.
The conventional gallium nitride compound semiconductor light emitting device has the following problems.
(1) Al.sub.z Ga.sub.1-z N (0&lt;Z.ltoreq.1) has a hardness higher than that of Al.sub.2 O.sub.3 or GaN, and this relationship tends to increase as the Al ratio is increased. Consequently, when the n-type clad layer 4 or the p-type clad layer 6 formed of Al.sub.z Ga.sub.1-z N having a thickness less than that of the sapphire substrate 1 is provided above the sapphire substrate 1, dislocations or cracks are generated in the n-type clad layer 4 or the p-type clad layer 6 in a concentrated manner by the lattice mismatch between the sapphire substrate 1 and the n-type clad layer 4 or the p-type clad layer 6. The generation of these dislocations or cracks becomes more significant as the Al ratio is increased.
When the n-type clad layer 4 is provided on the n-type contact layer 3, a larger thickness or a higher Al ratio of the n-type clad layer 4 is likely to generate dislocations or cracks in the n-type clad layer 4. The reason for this is that Al.sub.z Ga.sub.1-z N has a lattice constant less than that of GaN and this relationship tends to increase as the Al ratio is increased. The dislocations or cracks generated in the n-type clad layer 4 spread throughout the entirety of the gallium nitride compound semiconductor light emitting device, resulting in deterioration in the characteristics of the device.
(2) Al.sub.z Ga.sub.1-z N (0&lt;Z.ltoreq.1) has a thermal expansion coefficient greater than that of GaN, and this relationship tends to increase as the Al ratio is increased. Consequently, even though the crystallinity of the n-type clad layer 4 and the p-type clad layer 6 is satisfactory during crystal growth, a tensile strain is applied to the n-type clad layer 4 and the p-type clad layer 6 when the temperature is lowered from the crystal growth temperature to room temperature. As a result, dislocations or cracks are generated in the gallium nitride compound semiconductor light emitting device, with the generation of cracks becoming significant especially as the Al ratio is increased.
When the thickness of the n-type clad layer 4 and the p-type clad layer 6 is reduced in order to avoid the above-described problems, the light is not sufficiently confined in the active layer 5.
Due to the above-described problems, the conventional gallium nitride compound semiconductor light emitting device involves a defect density of about 10.sup.9 cm.sup.-2 and crack generation, which reduces the light emitting efficiency. Consequently, the conventional gallium nitride compound semiconductor light emitting device has inferior characteristics, represented by a threshold current of as high as about 60 mA.